Generally, an electric rolling stock is configured to take in electric power from a wire or a third rail by a power collector and run by driving a motor by using the collected power. In recent years, performance of electric-power storage elements such as a secondary battery and an electric double-layer capacitor has been improved. Therefore, there has been a development of a system that runs by driving a motor by concurrently using electric power of these electric-power storage elements mounted on an electric rolling stock.
Such a system includes an electric-power storage device configured to obtain a predetermined voltage by connecting plural electric-power storage elements as cells or modules of a secondary battery and an electric double-layer capacitor. A power converter to perform a control of electric charge/discharge is connected between an external power source and the electric-power storage device in order to electrically charge to and discharge from the electric-power storage device (for example, see Patent Documents 1 and 2).
There are various circuit modes as a power converting unit that constitutes a power converter. For example, as major ones of these circuit modes, there can be mentioned a DC-DC converter circuit having a function of performing electric charge/discharge by controlling power between a direct-current power source and a power storage unit at a predetermined value, a DC-AC inverter circuit having a function of electrically charging to an electric-power storage device by converting regenerative power generated from an alternating-current motor into direct current power and driving the alternating-current motor by converting direct current power of the electric-power storage device into alternate current power, and an AC-DC converter having a function of performing electric charge/discharge by controlling power between an alternating-current power source and an electric-power storage device at a predetermined value.
The power converter described above is generally configured to have a voltage detector (hereinafter, “first voltage detector”) that detects a voltage at a connection point between a power converter and an electric-power storage device and to perform an electric charge/discharge control of the electric-power storage device based on a voltage detected by this first voltage detector.
An electric-power storage element used in the electric-power storage device has a characteristic that a terminal voltage changes depending on an electric charge amount (SOC: State of Charge), and has a characteristic that the terminal voltage becomes higher when the SOC is higher. Therefore, a power converter that includes the electric-power storage device is set with a permissible maximum voltage that becomes an upper limit and a permissible minimum voltage that becomes a lower limit in a terminal voltage of each electric-power storage element in order to prevent damage due to an over electric discharge from and an over electric charge to each electric-power storage element. The power converter controls to reduce a charging current such that a voltage detected by the first voltage detector does not exceed the permissible maximum voltage, and also controls to reduce a discharging current such that the detected voltage does not become lower than the permissible minimum voltage.
Meanwhile, respective electric-power storage elements incorporated in the electric-power storage device is provided with a voltage detector in respective cell units or module units. The electric-power storage device that includes respective electric-power storage elements is generally configured to have a function of detecting and monitoring a voltage of respective electric-power storage elements and notifying an abnormality to a higher-order system by outputting an indication of the state to the system when a detected voltage exceeds or becomes lower than a predetermined value, and a monitoring function called a cell monitor or the like detecting an occurrence of a variation in voltages between cells or between modules to equalize the voltages such that the variation is suppressed.
The first voltage detector that detects a voltage at a connection point between the first voltage detector and the electric-power storage device is required to have a performance that can withstand a high voltage such that the first voltage detector can detect a voltage of about several hundred to one thousand volts. A main object of this voltage detector is to control a voltage of the power converter at a high speed. Although a response delay of an input/output is as small as 0.3 microsecond, an error of the voltage detection precision is as high as about ±3%. Generally, a voltage detector that can detect a high voltage tends to have poor detection precision because its internal insulating unit and the like are complex.
Besides, a voltage detector (hereinafter, “second voltage detector”) that monitors a voltage of an electric-power storage element detects a voltage of several to several tens of volts as a cell voltage or a module voltage. A main object of this voltage detector is to protect respective cells or modules from an over electric discharge and an over electric charge by monitoring the voltage of respective cells or modules in high precision. Therefore, although a response delay of an input/output is as large as several tens of microseconds, an error of the detection precision of a voltage is as high as about ±0.3%.
An example in which a power converter electrically charges to an electric-power storage element is explained below. When the SOC of the electric-power storage element is low and when a terminal voltage has a sufficiently apart from its upper limit value, a constant-current electric charge (CC (Constant Current) electric charge) of electrically charging to the electric-power storage element at a constant current is performed. When the SOC increases and when the terminal voltage reaches approximately to a permissible maximum voltage, the CC electric charge is switched to constant-voltage electric charge (CV (Constant Voltage) electric charge), such that the terminal voltage does not exceed the upper limit value. Electric charge is continuously performed by reducing a current such that a voltage of the electric-power storage element is maintained at a predetermined value while not exceeding the permissible maximum voltage.    Patent Document 1: Japanese Patent Application Laid-open No. 2007-274756    Patent Document 2: Japanese Patent Application Laid-open No. 2006-176057